This invention relates to threaded joints for oil well pipes used for the exploration and production of natural gas and crude oil beneath the ground, and particularly to a joint suitable for use in deep wells and in severe corrosive environments.
Today, threaded joints are widely used as a means for connecting oil well pipes used in the exploration and production of natural gas fields and crude oil fields, which may be at a depth of thousands of meters. Threaded joints for oil well pipes include integral types, in which a pin formed on one end of a tube is connected to a box formed on one end of another tube, and coupling types in which a coupling having a box formed on each end is used to connect pipes having pins formed on both ends.
Oil well pipes are installed beneath the ground in wells and are used under high pressures and high loads and is bad environments. Therefore, various capabilities are demanded of such threaded joints, including (1) that they be able to withstand axial tensile forces due to the weight of the pipes to which they are connected, (2) that they be able to withstand the internal pressure from fluids inside them, (3) that they do not corrode due to the fluids inside them, and (4) that they be capable of being reused dozens of times. In recent years, wells have tended to become deeper, so the above requirements have been become more stringent.
There have been many proposals for threaded joints for oil well pipes which can satisfy such demands.
FIG. 7(a) is a cross-sectional view showing two pipes 10 connected together by a coupling 20, and FIG. 7(b) is an enlarged view of a portion of FIG. 7(a). A seal is formed by contacting a tapered, seal-forming unthreaded portion 13 on the end of an externally-threaded pin 11 formed on the end of each pipe 10 and a tapered seal-forming unthreaded portion 23 on the inside of an internally-threaded box 21 formed on the inside of the coupling 20. 12 indicates a male thread portion and 22 indicates a female threaded portion. By forming a metal-to-metal seal in this portion, the leak-tightness against the internal pressure of fluids within the oil well pipes is increased.
An unthreaded portion 14 formed on the end of unthreaded portion 13 of the pin 11 is abutted against an unthreaded portion 24 formed inside of unthreaded portion 23 of the box 21 to form a torque shoulder. Due to the abutment between these portions, the make-up torque is adjusted to a suitable value so that a high contact surface pressure producing excessive plastic deformation is not generated.
Many proposals have been made for the shapes of the threads used in these joints, such as those described below.
The buttress threads shown in FIGS. 2(a) and 2(b) are prescribed by Standard 5B of the API (American Petroleum Institute). FIG. 2(a) shows the shape of the buttress threads, and FIG. 2(b) shows the state of contact between the male threads and female threads at the time of make-up. In FIGS. 2(a) and 2(b), the upper portion corresponds to an internally-threaded box portion 21 and the lower portion corresponds to an externally-threaded pin portion 11. Due to the threaded engagement during make-up, load flanks 1 are formed by a male load flank 1a and a female load flank 1b, stab flanks 2 are formed by a male stab flank 2a and a female stab flank 2b, crest surfaces 3 are formed by a male crest surface 3a and a female crest surface 3b, and root surfaces 4 are formed by a male root surface 4a and a female root surface 4b.
FIG. 1 illustrates the contact between the thread surfaces in a threaded joint according to this invention. It shows the load flank angle .alpha. and the stab flank angle .theta. of a buttress thread which influence the performance of a threaded joint. The flank angle is a positive or negative angle measured with respect to line X-Y or X'-Y' extending perpendicular to an unillustrated pipe axis. The load flank angle .alpha. is a negative angle measured in the counterclockwise direction, and the stab flank angle .theta. is a positive angle measured in the counterclockwise direction.
An API buttress thread has a load flank angle of 3 degrees and a stab flank angle of 10 degrees. As shown in FIG. 2(b), at the time of make-up, there is thread contact along the load flanks 1, no contact along the stab flanks 2, and contact along at least one of the crest surfaces 3 and the root surfaces 4. The dimensional tolerances for API standards allow a gap of from 0.03 to 0.19 mm between the stab flanks 2.
There also exists a thread shape referred to as an improved buttress thread, as shown in FIG. 3(a). This thread has a load flank angle of 0 degrees and a stab flank angle of 45 degrees. As shown in FIG. 3(b), at the time of make-up, there is contact along the load flanks 1 and the stab flanks 2, and gaps exist along the crest surfaces 3 and the root surfaces 4. In FIG. 3(b), the same parts as in FIG. 2(b) are indicated by the same reference numerals.
Threaded joints using the above-described thread shapes have the following problems.
Although the frequency of occurrence is not high, there are cases in which a compressive force acts in the axial direction of pipes. When a tensile load is subsequently applied due to the weight of the pipes, the contact surface pressure in the seal portion and the torque shoulder of the API buttress thread of FIG. 2 decreases, and in extreme cases, a gap develops. This phenomenon decreases the leak-tightness with respect to fluids within the pipes, and permits the internal fluids to easily leak and penetrate into the gaps, and a great deal of corrosion occurs in the gaps.
Furthermore, when this phenomenon is occurring, if for some reason a torque acts on the joint in the loosening direction, the joint can easily become disconnected, and there is the danger of the oil well pipes falling into the well. In this case, it is extremely difficult to reconnect the oil well pipes within the well, and in the worst case, it is necessary to abandon an oil well which is in the development stage or the production stage. This problem becomes more severe as the depth of a well increases, because the tensile forces due to the weight of the pipes increase. These phenomena occur due to there being a relatively wide gap at the stab flanks at the time of make-up, so when a compressive force is applied, the threads are not subjected to the compressive load until this gap disappears, and compressive loads concentrate in the seal portion and torque shoulder. In these portions, large plastic deformation develops. Therefore, when tensile forces subsequently act, the contact surface pressure in these portions is no longer the same as that before deformation and greatly decreases, and the maintaining torque of the joint decreases.
In the improved buttress thread shown in FIG. 3, there are cases in which the thread surfaces are damaged by as few as 10 repeated make-ups. This damage is not severe enough to be described as seizure, but abrasion damage is evident. The cause of this damage is that both the load flanks and the stab flanks are in a contacting state at the time of make-up. By increasing the flank angle of the stab flanks, the contact surface pressure at the stab flanks can be significantly decreased, but the contact surface pressure is still high. Accordingly, if the joint undergoes further continuous use, there is the danger of seizure developing with this damage as a starting point. In order to avoid this danger, it is necessary to machine the threads to modify their shape, which leads to the increased costs.